Python decorators in classes

Question

Hi,

can one write sth like:

class Test(object):
    def _decorator(self, foo):
        foo()

    @self._decorator
    def bar(self):
        pass

This fails: self in @self is unknown

I also tried:

@Test._decorator(self)

which also fails: Test unknown

If would like to temp. change some instance variables in the decorator and the run the decorated method, before changing them back.

Thanks, HC

Answer 1

What you're wanting to do isn't possible. Take, for instance, whether or not the code below looks valid:

class Test(object):

    def _decorator(self, foo):
        foo()

    def bar(self):
        pass
    bar = self._decorator(bar)

It, of course, isn't valid since self isn't defined at that point. The same goes for Test as it won't be defined until the class itself is defined (which its in the process of). I'm showing you this code snippet because this is what your decorator snippet transforms into.

So, as you can see, accessing the instance in a decorator like that isn't really possible since decorators are applied during the definition of whatever function/method they are attached to and not during instantiation.

If you need class-level access, try this:

class Test(object):

    @classmethod
    def _decorator(cls, foo):
        foo()

    def bar(self):
        pass
Test.bar = Test._decorator(Test.bar)

Answer 2

Would something like this do what you need?

class Test(object):
    def _decorator(foo):
        def magic( self ) :
            print "start magic"
            foo( self )
            print "end magic"
        return magic

    @_decorator
    def bar( self ) :
        print "normal call"

test = Test()

test.bar()

This avoids the call to self to access the decorator and leaves it hidden in the class namespace as a regular method.

>>> import stackoverflow
>>> test = stackoverflow.Test()
>>> test.bar()
start magic
normal call
end magic
>>>